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Explain Statements Using Index Merge

IndexMerge is a method introduced in TiDB v4.0 to access tables. Using this method, the TiDB optimizer can use multiple indexes per table and merge the results returned by each index. In some scenarios, this method makes the query more efficient by avoiding full table scans.

mysql> EXPLAIN SELECT * from t where a = 1 or b = 1;
+-------------------------+----------+-----------+---------------+--------------------------------------+
| id                      | estRows  | task      | access object | operator info                        |
+-------------------------+----------+-----------+---------------+--------------------------------------+
| TableReader_7           | 8000.00  | root      |               | data:Selection_6                     |
| └─Selection_6           | 8000.00  | cop[tikv] |               | or(eq(test.t.a, 1), eq(test.t.b, 1)) |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo       |
+-------------------------+----------+-----------+---------------+--------------------------------------+
mysql> set @@tidb_enable_index_merge = 1;
mysql> explain select * from t use index(idx_a, idx_b) where a > 1 or b > 1;
+--------------------------------+---------+-----------+-------------------------+------------------------------------------------+
| id                             | estRows | task      | access object           | operator info                                  |
+--------------------------------+---------+-----------+-------------------------+------------------------------------------------+
| IndexMerge_16                  | 6666.67 | root      |                         |                                                |
| ├─IndexRangeScan_13(Build)     | 3333.33 | cop[tikv] | table:t, index:idx_a(a) | range:(1,+inf], keep order:false, stats:pseudo |
| ├─IndexRangeScan_14(Build)     | 3333.33 | cop[tikv] | table:t, index:idx_b(b) | range:(1,+inf], keep order:false, stats:pseudo |
| └─TableRowIDScan_15(Probe)     | 6666.67 | cop[tikv] | table:t                 | keep order:false, stats:pseudo                 |
+--------------------------------+---------+-----------+-------------------------+------------------------------------------------+

In the above query, the filter condition is a WHERE clause that uses OR as the connector. Without IndexMerge, you can use only one index per table. a = 1 cannot be pushed down to the index a; neither can b = 1 be pushed down to the index b. The full table scan is inefficient when a huge volume of data exists in t. To handle such a scenario, IndexMerge is introduced in TiDB to access tables.

IndexMerge allows the optimizer to use multiple indexes per table, and merge the results returned by each index to generate the execution plan of the latter IndexMerge in the figure above. Here the IndexMerge_16 operator has three child nodes, among which IndexRangeScan_13 and IndexRangeScan_14 get all the RowIDs that meet the conditions based on the result of range scan, and then the TableRowIDScan_15 operator accurately reads all the data that meets the conditions according to these RowIDs.

For the scan operation that is performed on a specific range of data, such as IndexRangeScan/TableRangeScan, the operator info column in the result has additional information about the scan range compared with other scan operations like IndexFullScan/TableFullScan. In the above example, the range:(1,+inf] in the IndexRangeScan_13 operator indicates that the operator scans the data from 1 to positive infinity.

Note
  • The Index Merge feature is enabled by default from v5.4.0. That is, tidb_enable_index_merge is ON.

  • You can use the SQL hint USE_INDEX_MERGE to force the optimizer to apply Index Merge, regardless of the setting of tidb_enable_index_merge. To enable Index Merge when the filtering conditions contain expressions that cannot be pushed down, you must use the SQL hint USE_INDEX_MERGE.

  • Index Merge supports only disjunctive normal form (expressions connected by or) and does not support conjunctive normal form (expressions connected by and).

  • Index Merge is not supported in tempoaray tables for now.

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